Unless state prisons (as opposed to local jails) are housing a LOT of people with > 1 year sentences for misdemeanors of violence, the nearly three-quarters of a million point-in-time prison population for violent crimes looks very funny compared to the cumulative NICS total of less than half of a million.

BTW, the NICS Index is not a record of denied firearms purchase attempts; the NICS Index is the source used to determine whether a purchase is denied.

ADDED: Illegal aliens are, by definition, prohibited persons; all ICE activity is dumped into NICS to make the numbers look impressive.